Travel trailers are often provided with an auxiliary compartment, called a slideout box, which is mounted in a vertical outer wall of the trailer so that when the trailer is in camp position, the box can be slid outwardly giving added room inside the trailer proper. When the trailer is to be moved, the box is slid inwardly so that the outer wall of the box is substantially flush with the wall in which the box is mounted. The movement of the box is effected by a drive motor which moves the box from its inner position within the body of the trailer, to its outer position where it projects beyond and outside the trailer wall. The drive motor may be relatively small since the movement of the slideout box does not need to be effected rapidly. However, when the box is in its innermost position, the drive motor is of insufficient torque to keep the slideout box firmly sealed against the trailer wall.
To the end that the box may be sealed firmly against the trailer wall when in its inner position, the present invention provides a locking member for holding the box firmly inward whereby it may be well sealed against the outer wall.